Memory devices provide data storage for electronic systems. One type of memory is a non-volatile memory known as flash memory. A flash memory is a type of EEPROM (electrically-erasable programmable read-only memory) that may be erased and reprogrammed in blocks. Many modern personal computers have BIOS stored on a flash memory chip. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in wireless electronic devices because it enables the manufacturer to support new communication protocols as they become standardized, and to provide the ability to remotely upgrade the device for enhanced features.
A typical flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. The cells are usually grouped into blocks. Each of the cells within a block may be electrically programmed by charging a charge storage gate of the cell. The charge may be removed from the charge storage gate by a block erase operation. Data is stored in a cell as charge in the charge storage gate.
NAND is a basic architecture of flash memory. A NAND cell unit comprises at least one select gate coupled in series to a serial combination of memory cells (with the serial combination being commonly referred to as a NAND string). The gates of the NAND string have traditionally been single level cells (SLCs), but manufacturers are transitioning to utilization of multilevel cells (MLCs) for gates of NAND strings. An SLC stores one bit of memory, whereas an MLC stores two or more bits of memory. Accordingly, memory can be at least doubled by transitioning from SLCs to MLCs.
MLCs differ from SLCs in the programming of the devices. Specifically, a device may be programmed as an SLC if the device is programmed to have only two memory states (0 or 1), with one of the memory states corresponding to one level of stored charge (for example, corresponding to the fully charged device) and the other corresponding to another level of stored charge (for example, corresponding to the fully discharged device). Alternatively, the device may be programmed as an MLC having two bits of memory if the device is programmed to have four memory states. The memory states may be designated as the 00, 01, 10, and 11 memory states, in order from lowest stored charge (for example, fully discharged) to highest stored charge (for example, fully charged). Accordingly, the 00 state corresponds to a lowest stored charge state, the 11 state corresponds to a highest stored charge state, and the 01 and 10 states correspond to first and second intermediate levels of stored charge.
Non-volatile memory cells comprise sub-structures which include a control gate, a charge storage gate (which may be referred to as a floating gate), an intergate dielectric between the control gate and charge storage gate, and a tunnel dielectric between the charge storage gate and an underlying substrate. The charge storage gate may correspond to a material within which charge is mobile (for instance, silicon or conductively-doped silicon), or may correspond to a charge trapping material (for instance, silicon oxynitride). Charge trapping materials offer some advantages relative to other charge storage materials in that they may be formed relatively thin, and therefore may be advantageous for future flash device scaling. However, charge trapping materials formed by conventional methods are difficult to utilize in MLC devices. Specifically, it is desired for MLC devices to have a voltage difference between the lowest stored charge state and the highest stored charge state of at least about 8 volts in order to have four distinct programmable states. It is difficult to obtain such voltage difference with charge-trapping materials formed by conventional methods while simultaneously maintaining desired charge retention characteristics (such as charge retention characteristics associated with 10 year data retention requirements).